/Engine/Plugins/Runtime/ReplicationGraph/Source/Public/ReplicationGraph.h
This setting determines the number of frames from the last Gather call when we consider a node obsolete. The default value of 0 disables this condition. Enable this to reduce the memory footprint of the RepGraph and optimize the exponential cost of dormancy events (the more each connection moves around the grid cell the more each dormancy events is listened to). Inversely if clients go back and forth across the same cell multiple times the cost of the first Gather of newly created cell is very high (it rechecks all dormant actors in the cell before removing them). This means this optimization is ideal only if you know the clients can't go back to previously visited cells, or that they do so very infrequently.
static void SetNumFramesUntilObsolete ( uint32 NumFrames )